The present invention comprises a joint which connects a rod and a fitting, wherein the rod and fitting are subjected to high-tension loads. The invention is especially useful in constructing a sucker rod for oil wells. The invention may be used, however, in many other applications, such as in pre-stressed concrete, and in tension members for bridges.
A sucker rod is a long rod formed of a plurality of sections, capable of connecting a pump, located at the bottom of an oil well, with a lifting device, or pump jack, located at the surface. Each section may include a rod portion and a fitting portion, each rod being inserted into a fitting. In practice, a sucker rod comprises a long chain of rods and fittings and couplings. Because some wells may be deeper than 16,000 feet, the sucker rod must have very high tensile strength, so that the rod does not break under its own weight plus that of the fluid, as it is pulled up by the pump jack, and pulled down by gravity. The problem of designing a sucker rod having the necessary tensile strength is made more difficult by the fact that the rod typically must fit within a tubing of relatively small diameter.
Various sucker rods have been proposed in the prior art, as exemplified by U.S. Pat. Nos. 4,205,926, 4,430,018, and 6,193,431. The disclosures of these patents are hereby incorporated by reference.
While the above-cited patents provide useful solutions, the patented devices have disadvantages. Among other things, some of the prior art uses rods that are made of stranded wire. A stranded rod, by definition, has less than optimal tensile strength, because the strands do not occupy the entire cross-sectional area of the rod, and the filaments are not aligned exactly parallel to the rod axis.
In general, the prior art has not developed a sucker rod having sufficient tensile strength, while occupying a minimal cross-sectional area. There is still a need for a sucker rod which is stronger than sucker rods of the prior art, which incorporates advanced materials that are lighter than metal, and which still fits within a narrow tubing bore.
The present invention provides a joint which exhibits very high tensile strength, while minimizing the cross-sectional area required. The joint of the present invention can therefore be used to construct longer, lighter sucker rods which can still lift a useful payload of fluid. It can also be used in many other applications requiring the assembly of rods capable of carrying large tensile loads. However, the joint of the present invention is not intended to bear significant compression loads.